A lens acts by bending the light wavefront falling thereon, i.e., the light beam coming out of the lens exhibits a phase offset, different from point to point of the lens surface, with respect to the incoming light beam to the lens; generally speaking, this happens because of the refraction of light on the surfaces of the lens, which depends both on the refraction index of the material and on the geometry of the lens surfaces. Thus, these both parameters can be acted upon in order to achieve a desired wavefront.
An example of a lens having a variable geometry surface is the classical Fresnel lens, which is an aggregate of differently shaped annuli from which the redundant thickness causing a phase offset multiple of 2π has been removed.
Another example of a lens presenting a variable refraction is a progressive ophthalmic lens, in which the power varies spatially across the lens surface. This is achieved by varying the refraction index or the curvature of the lens surfaces on each point.
In these examples the phase offset is different in different portions of the lens, i.e., the focal length presents a spatial variation across the lens. But it would be also interesting to obtain a lens having a focal length which varies with time. This idea leads to the concept of a programmable lens, which could be defined as a lens the focal length of which can be made to vary with time in a controlled manner. More generally, in a programmable lens the phase offset varies not only spatially across the lens surface but it can be made to vary with time too.
Generally speaking, lenses belong to a kind of optical devices named Spatial Light Modulators, or SLM, although the term SLM is normally applied to electro-optical devices. The modulation referred to can be an amplitude modulation or a phase modulation, though the present invention relates mainly to phase modulators.
The surface of an electro-optical SLM (in the following simply termed SLM) can be divided in portions that may independently adjust or modulate the amplitude or the phase of a light beam falling onto them; said portions are controlled by electrodes. Such a SLM can thus be suitable to work as a programmable lens.
An SLM can modulate the light by reflection or by transmission. For ophthalmic lenses it is, in principle, more convenient a transmission design, but a reflection device could also be interestingly applied to ophthalmic lenses.
One of the SLMs most useful in order to vary the focal length of the lens is the nematic liquid crystal display device (LCD or N-LCD). A LCD comprises a matrix of small cells or pixels of liquid crystal, each pixel having electrodes that allow to modify the optical behaviour of the liquid crystal housed in the cell constituting the pixel by applying a voltage. These modifications result in variations of the optical path that cause a phase offset which is equivalent to that caused by the variations of the refraction index or the curvature in conventional refractive lenses. By using such a device, and by generating a phase distribution that is equivalent to a conventional lens (mono-focal, bi-focal, progressive, . . . ), a so called diffractive lens is obtained.
In diffractive optics, in contrast to refractive optics, it is not enough to represent the light by straight rays, but diffraction phenomena related to the wave nature of light are to be taken into account.
A drawback of diffractive lenses is that a change in the wavelength of the incoming light results in a change of the focal length, more acutely than with refractive lenses. This means that when light is not monochromatic, or almost monochromatic, then the image shows an important aberration, called chromatic aberration. This aberration becomes apparent in that the lens has a different power for any wavelength.
Said chromatic aberration can be classified as longitudinal and transversal. Longitudinal chromatic aberration is related to the fact that the lens focalizes on different planes along the revolution axis of the lens depending on the wavelength. But even if the focal plane is the same for all wavelengths (or at least for the considered wavelengths), it may happen that the intensity distribution delivered by the optical system on the image plane depends on the wavelength. It is then said that the systems presents a transversal chromatic aberration, which becomes apparent in an iridescence on the image.
Patent U.S. Pat. No. 4,601,545 discloses a variable power lens comprising a LCD controlled by a plurality of electric potentials in the form of an addressable matrix, in order to provide a gradient for the refraction index of the lens. In operation, the magnification of an image varies with the gradient of the electric field applied upon the aperture of the lens, and the power of the lens varies by varying the refraction index of the LCD. This document does not mention any correction of the chromatic aberration.
The paper “Achromatic diffractive lens written onto a liquid cristal display” (Márquez et al., OPTICS LETTERS, Vol. 31, No. 3, Feb. 1, 2006), discloses a programmable diffractive lens having the same focal length for several wavelengths simultaneously. Said lens is not designed for ophthalmic applications and with it the wavelength is not selected by means of spectral filters, whereby multiple focalizations are obtained on different planes and the chromatic aberration is not compensated.
A prototype of a programmable lens has recently been developed in the University of Arizona, said prototype being based on an LCD controlled by some electrodes arranged as concentric annuli. Such a device can only reproduce mono-focal or bi-focal lenses and does not compensate chromatic aberration. An initial description of the device can be found in “Optics & laser Europe” May 2006, Issue 139, page 11.